1. Field of the Invention
This invention relates to a process for removing the residual tensile welding stresses in the inner layer of the weld metal and heat-affected zone (HAZ) of steel piping elements that have been butt-welded to each other end-to-end by means of a circumferential weld.
2. Description of the Prior Art
When piping is butt-welded together by means of a circumferential weld, significant residual tensile welding stresses are produced in the weld metal and in the heat-affected zone of said piping. These tensile stresses tend to enhance stress corrosion cracking in the welded region and the resulting crack propagation in the weld metal and in the heat-affected zone of such piping.
Stress corrosion cracking in stainless steel piping has been a serious drawback in boiling water nuclear reactor (BWR) plants in the United States and elsewhere in the world. Mitigating remedies have included hydrogen addition to the water in the pipes, the use of improved welding techniques and the use of better materials in the preparation of the steel piping. However, the occurrence of cracks have not been fully eliminated or significantly reduced in BWR plants.
Induction heating stress improvement (IHSI) is one method currently being used to improve residual welding stresses in pipes and shells. However, the IHSI process cannot induce net compressive forces through the weld in either the axial or circumferential direction. Equilibrium of the axial compressive stresses due to the IHSI process induces axial tensile stresses such that the net axial force in every cross-section is zero. Indeed, through-the-wall bending stresses are induced during the IHSI process. Weld defects, such as inclusions, porosity, lack of fusion, hot tears, etc., and stress corrosion or fatigue cracks, are therefore subjected to high tensile stresses during the process. The resulting crack opening, fatigue cracking and creep crack propagation increases exposure to the corrosive media and reduces the remaining strength and endurance of the piping and makes it more susceptible to further cracking and leaking. Our Improved Mechanical Stress Improvement Process is an improved process in that high tensile stresses are not induced in the welded region of the piping during the claimed process, thus eliminating the potential deleterious effects of the IHSI tensile stresses.
Numerous procedures have been employed to improve the physical and/or mechanical properties of materials that have been welded to each other. Verdier in U.S. Pat. No. 3,625,568 and British Pat. No. 1,217,803 relieves tensile stresses in a welded joint by subjecting the weld to a compressive force. In U.S. Pat. No. 4,018,634 Fencl strain hardens a pipe having a longitudinal seam by decreasing the diameter of the pipe along its entire length by at least 1.5 percent and thereafter subjecting the treated pipe to a heat operation. Beatovic et al in U.S. Pat. No. 4,342,609 and European Pat. No. A1 0014158 lower tensions arising from a welding operation by detonating an explosive charge placed over the weld area. Howd et al in British Pat. No. 1,235,106 and Loosemore et al in British Pat. No. 2,071,552 also use an explosive charge. Avesta in British Pat. No. 1,000,133 subjects a vessel containing cold-worked welded plates, while at a temperature below the recrystallization temperature of the steel plates, to an internal fluid pressure to obtain a substantially uniform strength in the vessel. Davies et al in British Pat. No. 1,097,571 subjects a tube containing a longitudinal seam to swaging to reduce its wall thickness and then to heat treatment to effect recrystallization of the structure. Malik in British Pat. No. 2,048,146 improves the physical properties of weldments by heating the same to a very high temperature while simultaneously applying substantial isostatic pressure thereto. In Russian Pat. No. 474,564 to PATM low-alloy steel welds have been heated to their austenitic temperatures and submitted to deformation. UDOD in Russian Pat. No. 779,442 subjects a welded structure to peening by directing shot blasts at an angle of 70.degree..